Hybrid vehicle fuel systems may include a fuel vapor canister packed with adsorbent for adsorbing fuel tank vapors. The fuel tank vapors adsorbed may include refueling vapors, diurnal vapors, as well as vapors released during fuel tank depressurization. By storing the fuel vapors in the canister, fuel emissions are reduced. At a later time, when the engine is in operation, the stored vapors can be purged into the engine intake manifold for use as fuel. However, in hybrid vehicle systems, due to the limited use of engine operation, there may not be sufficient time to purge all the fuel vapors to the canister. Accordingly, various approaches have been developed to expedite release of fuel vapors from a fuel system canister, particularly in hybrid vehicles.
One example approach is shown by Atsumi et al. in US 2010/0094493. Therein, a fuel vapor canister of a hybrid electric vehicle system is configured with a canister heater. The heater is operated when the vehicle is charged from an external source, so as to reduce the usage of charge from a system battery. Specifically, the canister is preheated by the heater while the vehicle is being charged with power from an external source and before an engine is operated.
However, the inventors have identified potential issues with such an approach. As one example, the approach of Atsumi does not allow for canister temperature control. Applicants have recognized that heating the canister to different temperatures at different operating conditions may improve the purging efficiency of the canister. For example, at higher temperatures (e.g., higher canister temperatures and/or higher ambient temperatures), the desorbing efficiency of the canister may be elevated and therefore substantial canister heating may not be required. As another example, at lower temperatures (e.g., lower canister temperatures and/or lower ambient temperatures), the adsorbing efficiency of the canister may by elevated and therefore substantial canister heating may be required.
In one example, some of the above issues may be at least partly addressed by a method for a hybrid vehicle system comprising: during vehicle travel, in response to an upcoming transition from an engine-off mode to an engine-on mode of vehicle operation, heating a fuel system canister before starting an engine. In this way, canister purging efficiency in a hybrid electric vehicle is improved.
As an example, a hybrid vehicle system may be configured with an engine and a system battery. A fuel system coupled to the engine may include a canister for storing fuel tank vapors, the canister configured with a heater. While the vehicle is operated in an engine-off mode, where the vehicle is propelled with energy from the system battery, vehicle operating conditions may be monitored to better anticipate an imminent transition to an engine-on mode. For example, one or more vehicle operating conditions such as battery state of charge, fuel tank fuel level, vehicle speed, torque demand, ambient temperature, etc., can be monitored to determine if vehicle propulsion via the engine is likely. If an engine-on mode is imminent, a canister heater may be operated, using energy from the battery, to heat the canister to a threshold temperature while the vehicle is propelled using motor torque. The threshold temperature to which the canister is heated may be based on vehicle operating conditions (such as battery state of charge, fuel tank fuel level, vehicle speed, torque demand, ambient temperature, etc.) as well as the prevalent canister temperature. For example, as the ambient temperature increases, the canister may be heated to a lower threshold temperature. As such, at the higher ambient temperature, reduced canister heating may be required due to the higher (inherent) desorption of fuel vapors from the warmer canister. In comparison, as the ambient temperature decreases, the canister may be heated to a higher threshold temperature due to the higher (inherent) absorption of fuel vapors into the cooler canister. Canister heating settings (e.g., power setting, duration of heating, et.) may be adjusted so that the canister is heated to the threshold temperature before the engine is started. During the subsequent engine restart, a canister purge valve may be opened and fuel vapors desorbed from the heated canister may be rapidly purged to the engine intake.
In this way, canister temperature control is improved. By adjusting canister temperature settings during vehicle travel based on an imminent transition to an engine-on mode, the impact of canister heating on the electric mode of vehicle operation is reduced. By heating the canister to a temperature based on vehicle operating conditions, fuel desorption is enabled more efficiently, improving vehicle battery usage and enabling canister purging to be performed using a smaller air mass. By heating the canister to a threshold temperature before the engine is restarted, canister purging can be enabled as soon as the engine is started. Furthermore, canister purging can be completed in a smaller amount of engine operation time. Overall, canister purging efficiency is improved without degrading the fuel economy of a hybrid vehicle system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.